General
The phenomenon of superconductivity was discovered in 1908 by Dutch Physicist Kamberlign Onnes, while studying the electrical resistance properties of pure mercury at very low temperatures. A superconducting material is one that when cooled below its critical transition temperature (Tc) will lose all it measurable electrical resistance. In 1933, Meissner and Oschenfield discovered that superconductors not only have zero electrical resistance, but also behave like perfect diamagnets. Superconductors are classified into two categories depending upon their magnetization properties. In an applied magnetic field, Type-I superconductors undergo a reversible thermodynamic transition from the perfectly diamagnetic superconducting state to the normal resistive state. Type II superconductors undergo two irreversible thermodynamic transitions. The first occurs at a lower critical field Hc1, and is a transition from a perfectly diamagnetic superconducting state to a “mixed” or vortex state. The second occurs at an upper critical field Hc2, and is a transition from the mixed state to the resistive normal state. In the mixed state, quantized units of magnetic field known as fluxoids are allowed to penetrate the superconducting material, while the bulk material maintains its diamagnetism. When a superconducting material is in its mixed state with fluxoids penetrating the material and a transport current is passed through the material, a Lorentz force is developed between the fluxoid and the transport current. If the fluxoid in not “pinned” to the superconducting material then it will move under this Lorentz force causing unwanted dissipation. A key to fabricating a practical superconducting is to have the “pinning” force large enough to withstand the Lorentz force from significant current flow. There are several known methods to increase pinning forces in superconductors each pertaining to the introduction of defects into the materials. Some known methods include physical defects, chemical defects, irradiation, etc, and can be found in prior artwork: U.S. Pat. No. 4,996,192 by Fleisher et al., 2) U.S. Pat. No. 5,034,373 by Smith et al., and U.S. Pat. No. 5,292,716 by Saki et al.
For any superconducting material there is a maximum or critical current density (Jc) that the material is able to conduct, a maximum or critical magnetic field (Bc) that can be applied, and a maximum or critical temperature (Tc) that the material can experience, without developing resistance. These three critical parameters of a superconductor are all interrelated and each play a crucial role in developing a practical material that can be used in real world applications. For example, in an externally applied magnetic field (H), the critical current density Jc (T, H) of a superconductor will decrease with increasing applied field. Similarly, the critical current density Jc (T, H) will decrease with increasing temperature up to the transition temperature Tc, where the material will revert back to its normal state.
High Temperature Superconductors and Low Temperature Superconductors
Until the 1986, all known superconducting materials had critical transition temperatures below ˜23 K. This class of superconductors is commonly referred to as Low Temperature Superconductors (LTS) and typically consist of certain metallic or inter-metallic compounds (e.g. Nb, Va, Hg, Pb, NbTi, Nb3Sn, Nb3Al, Nb3Ge, etc.). In 1986, a new class of materials based upon oxide superconductors was discovered. This class of materials had significantly higher transition temperatures. They are commonly referred to as High Temperatures Superconductor (HTS) with some examples including Re—Ba—CuO, Bi—Sr—Ca—Cu—O, (Bi, Pb)—Sr—Ca—Cu—O, Tl—Ba—Ca—Cu—O, and Hg—Sr—Ca—Cu—O.
Coated Conductors
Oxide based HTS materials tend to have strong spatial anisotropic critical current and critical magnetic fields, while most of the practical metallic/inter-metallic LTS materials tend to have isotropic critical current and critical magnetic field properties. The existence of this strong anisotropy in HTS materials has led the development of very specific fabrication methods, including the second generation coated conductors, which form the basic current carrying element of one embodiment of this invention. Second generation coated conductors use external means (i.e. not natural crystal structure) to introduce texturing to a substrate template. Films of non-superconducting buffer layers and superconducting layers are deposited in a highly controlled temperature and pressure environment onto this textured substrate template for the specific purpose of subsequently growing HTS films with a high degree of in-plane crystal orientation. There are several known methods used to fabricate second generation HTS coated conductor including: rolling assisted bi-axial textures substrates (RABiTS), ion assisted beam deposition (IBAD), inclined substrate deposition (ISD), photo assisted chemical vapor deposition (PACVD), etc.
Until 1996, most HTS films were fabricated using traditional thick and thin film techniques for use in high frequency electronic device applications. Typical thick film techniques include sol-gel, dip coating, spin coating, etc. Typical thin film techniques include rf/dc sputtering, co-evaporation, CCVD, CVD, PVD, laser ablation, etc. Using these known film deposition techniques, very high quality HTS films with Jc>106 A/cm2 (77 K, self-field) were fabricated (see for example U.S. Pat. No. 5,231,074 by Cima et al). The primary reason for this success was that the HTS films were deposited on single crystal substrates that possessed a “natural” textured crystal structure orientation. Some typical single crystal substrates that have been used successfully to deposit texture HTS films are: sapphire (Al2O3), magnesium oxide (MgO), lanthanum aluminate (LaAlO3), strontium titinate (SrTiO3), as well as several others. The key to high quality HTS films once again being this natural highly oriented crystal structure template. By depositing the HTS films on highly oriented crystalline substrate templates, the HTS crystals themselves could grow in a highly textured format. With this high degree of crystal texture, HTS films will carry in excess of >106 A/cm2 at 77K, self-field. When HTS crystals are randomly aligned i.e. polycrystalline, they will have extremely low critical current densities. Low critical current densities are not useful in most real world device applications. For example, when HTS material is deposited on polycrystalline metallic substrates (e.g. Ag, Ni, or Ni alloy), the result is a very poor quality HTS film with very low Jc's. Although high quality, high Jc HTS films could be grown quite readily on rigid crystalline substrates for use in electronic device applications (e.g. cavities, high frequency filters, mixers, etc.), they could not be fabricated into long lengths, which are necessary for most electromagnet applications (e.g. motors, generators, magnets, transformers, cables, superconducting magnetic energy storage-SMES, Fault Current Limiters-FCL's etc.).
In 1996, researchers began to introduce thick/thin film deposition methods for fabricating long length coated conductors on flat (polycrystalline) metallic substrates. The metal of choice was typically hastelloy, Ni or one of its alloys, because of its ability to tolerate the high reaction temperature (>700° C.) necessary for HTS phase formation, yet remain mostly chemically inert. Typically, metals have a polycrystalline order and directly depositing HTS materials on them would result in poor quality, low Jc films. The key to fabricating high quality, high Jc material on polycrystalline metallic substrates was the imparting of an “external” texturing means to either the template itself (e.g. RABiTS) or imparting a texturing means by the deposition process itself (e.g. IBAD, ISD, PACVD). Several of the known methods for imparting texture to the HTS materials (IBAD, RABiTS, ISD, PACVD), are known to produce high quality, high Jc coated conductor. These external texturing techniques can be found in the prior artwork of: 1) Budai et al. (U.S. Pat. No. 5,968,877→October 1999), 2) Chu et al. (U.S. Pat. No. 5,906,964→May 1999), 3) Arendt et al. (U.S. Pat. No. 5,872,080→February 1999), and 4) Feenstra et al. (U.S. Pat. No. 5,972,847→October 1999. It is the use of the high quality, high Jc coated conductor that form the basic current carrying element of this device.
Potential Applications
Potential commercial and military applications of this novel stacked LTS/HTS magnet technology include: active denial systems for non-lethal standoff weapons, high-field insert coils for >1 G Hz NMR applications, laboratory-scale research magnets, magnetic bearings for flywheel energy storage and magnetic levitation, high power motor/generators, transformers, and inductive fault current limiters.